1. Field of the Invention
This invention relates to methods of forming calcium silicate and the resulting product.
2. Description of the Prior Art
Calcium silicate insulation is widely used, particularly for applications involving hot face temperatures above 800.degree. F. A number of processes for forming calcium silicate insulation products are known. A summary of certain United States patents disclosing such processes is contained in the prior art section of U.S. Pat. No. 4,238,240. In one prior art process for the formation of calcium silicate, calcium hydroxide (Ca(OH).sub.2) and powdered quartz (SiO.sub.2) are placed in an autoclave and mixed with water so as to form less than a given percent by weight (typically less than 7%) of the total weight of water in the autoclave. The contents of the autoclave are then heated to a selected temperature and pressure for a given period of time. The result is the formation of crystals of calcium silicate. Following the completion of the reaction which is determined when the calcium silicate crystals reach the desired size, (typically this reaction time is 55 minutes or greater) the reaction products are removed from the autoclave and further processed. In the process disclosed in U.S. Pat. No. 4,238,240, the reaction product is transferred from the autoclave to a holding vessel, the pressure in which is held beneath the pressure in the autoclave in a controlled manner during the transfer process. During the transfer process, the reaction products are sometimes passed through a heat exchanger where heat is extracted from the reaction products thereby to stabilize these products and to recover a portion of the energy used in heating the autoclave.
The solids content of the autoclave and the reaction time are two of the prime factors which determine the viscosity of the reaction products contained within the autoclave. Transfer of product from the autoclave to the holding vessel becomes progressively more difficult with increasing viscosity. For a given raw material formulation and processing conditions, viscosity increases with the solids content in the autoclave and also changes during the course of the chemical reaction. Studies show that the flow regime of the reaction constituents from the autoclave to the holding vessel is closely related to the pressure differential. It is common experience that the longer the reaction the more difficult it is to remove the reaction product in a controlled manner.
The economics of manufacturing calcium silicate are such that the greater the solid content of the reaction constituents in the autoclave, the more calcium silicate insulation of a given density can be fabricated from any given batch of reaction product. For given raw materials and processing conditions, the greater the solid content in the autoclave, the slower is the process of transferring the calcium silicate from the autoclave to the holding vessel and the greater the likelihood of flow blockage.
Following the transfer of the reaction product from the autoclave to a holding vessel, the reaction product is further modified, often by the addition of fibrous material to the reaction product and then is transferred to either filter presses or molds for the purpose of forming the calcium silicate reaction product into a shaped insulation.
It has also been proposed to add clay or the like to the reacted slurry of hydrated calcium silicates to overcome certain defects in molded calcium silicate products produced from the slurry. However, this method has been found unsatisfactory because it requires a longer molding time and the drying shrinkage of the molded product increases proportionately to the amount of additive such as clay. This method is discussed, for example, in British Patent Specification No. 1,462,242 filed Jan. 9, 1974. In the '242 specification, it is also disclosed that the reaction time required to form a calcium silicate hydrate by the hydrothermal reaction of a siliceous material with a calcareous material is shortened by adding caustic alkali to the reaction constituents. However, the '242 specification also points out that the degree of crystallinity of hydrated calcium silicates decreases in proportion to the amount of caustic alkali and that uniform crystals of calcium silicate hydrate could not be obtained. The resulting molded product from the slurry had increased drying shrinkage and very inferior resistance against high temperature.